Epitaxial nickel oxide (NiO) thin films have been grown by Metal Organic Chemical Vapor Deposition on AlGaN/GaN heterostructures. Critical growth parameters have been studied in order to optimize the deposition process of NiO films suitable for applications in GaN-based devices. In particular, a second generation precursor has been used as nickel source, namely the N,N,N′,N′-tetramethylethylenediamine adduct of nickel bis 2-thenoyl-trifluoroacetonate, using different deposition temperatures and oxygen flow values. Optimized operative conditions allowed the growth of epitaxial thin films which exhibited a permittivity of 11.7, close to the bulk value. The electrical characterization of the obtained epitaxial films pointed out to promising dielectric properties for AlGaN/GaN transistor technology.